Radiography

ABSTRACT

In a radiographic apparatus, a source of radiation is arranged to scan a planar spread of radiation in a plane about the body of a patient being examined. A plurality of detector devices is arranged to provide data representing the absorption of the radiation along a plurality of paths within the spread, for different orientations in the plane, for processing to determine a distribution of absorption coefficients for a planar slice of the body. The arrangement is such that a predetermined number of the detector devices are irradiated by the spread of radiation at any time and the actual devices irradiated change progressively as the scan progresses.

This is a continuation of application Ser. No. 946,644, filed on Sept.28, 1978 of Christopher Archibald Gordon LeMay for RADIOGRAPHY (U.S.Pat. No. 4,411,011 scheduled to issue on Oct. 18, 1983), which in turnis a continuation of Ser. No. 781,345 filed on Mar. 25, 1977 (nowabandoned), which in turn is a continuation of Ser. No. 668,518 filed onMar. 19, 1976 (now U.S. Pat. No. 4,031,395).

The present invention relates to radiographic apparatus of the kindarranged to provide a representation of the variation of absorption withposition across a planar slice of a body with respect to penetratingradiation.

In U.S. Pat. No. 3,946,234 there is described an apparatus for thatpurpose which includes a source of penetrating radiation arranged toprovide a fan-shaped spread of radiation lying in the plane of theslice. Suitable collimators are provided to define a plurality of pencilbeams from that spread and an array of detectors is arranged to measurethe intensity of each of those beams after passage through the body. Thedetectors are required to provide output signals indicative of theabsorption suffered by the radiation over a large number of pathsthrough the body. For that purpose the source and detectors arereciprocated in the plane of the slice and orbited about a common axisnormal to that plane. The output signals are processed by any suitablemethod, for example the convolution method described in U.S. Pat. No.3,924,129, to provide the desired representation.

Further developments of the apparatus is described in U.S. Pat. No.3,937,963 and U.S. application Ser. No. 544,799. According to thosespecifications the fan-shaped spread of radiation subtends an anglesufficient to include the whole region of interest in the plane of theslice so that a complete scan can be effected solely by orbiting thesource and detectors about the common axis.

It is an object of the present invention to provide an alternativeapparatus for the same purpose.

According to the invention there is provided an apparatus for examininga body by means of penetrating radiation including a source meansarranged to irradiate the body with a planar spread of said radiation,detector means arranged to detect the radiation to provide outputsignals, relating to absorption of the radiation by the body, forprocessing to provide a representation of the distribution of absorptionin a substantially planar section of the body and means adapted to scanthe planar spread of radiation in relation to the body so as toirradiate said section along a plurality of beam paths passing throughthe body from a plurality of directions, wherein said detector meanscomprises a plurality of detector devices of which a predeterminednumber are irradiated by said spread at any time and wherein thescanning means is arranged to traverse the spread x-rays along thedetector devices so as to change the irradiated devices progressively.

According to a feature of the invention the detector devices areinterconnected in groups of which not all devices are irradiated at anytime so that the outputs of all devices in any group are provided as oneoutput channel for the said processing.

In order that the invention may be clearly understood and readilycarried into effect, examples thereof will now be described withreference to the accompanying drawings of which,

FIGS. 1a and 1b show in simplified form, in end and side elevationrespectively, an apparatus incorporating the invention,

FIG. 2 shows a detector arrangement for one example of the invention,

FIG. 3 shows an arrangement for an alternative embodiment of theinvention and,

FIG. 4 shows in block diagrammatic form a circuit used for processingdata derived from the apparatus.

The apparatus is shown in a simplified form in end elevation in FIG. 1aand in side elevation in FIG. 1b and comprises a rotary member 1, whichis rotatable about an aperture 2 in which the body 3 of a patient to beexamined can be inserted. The body 3 shown in transverse section issupported on a suitably shaped bed 4 also shown in transverse section. Amaterial 5 having an absorption to the radiation similar to body tissueis positioned between the body 3 and the bed 4 to substantially excludeair from the gap therebetween and is extended partly about the body toprovide an approximately circular cross section to the radiation. Thebody is retained firmly in the desired position by means such as arestraining strap 6. If desired a more rigid arrangement such as thatshown in U.S. Pat. No. 3,946,234 can be used. Means for properlypositioning bed 4 may take any suitable form and are indicated generallyat 7. The rotary member 1 is rotatably mounted on a fixed frame 8,having an aperture at least commensurate with aperture 2. Member 1 isrotated by means of a gear wheel 9a journalled in frame 8 and drived bya motor 10. The gear wheel 9a engages gear teeth, not shown, formedaround the periphery of member 1. Other, non driven, gear wheels 9, alsojournalled in frame 1a, are also provided to properly support rotarymember, 1 and bearings 11 are provided to restrict axial motion. A lightsource/photocell device 12 fixed to main frame 18 co-operates with agraticule 13 to provide pulses indicative of the progress of the rotarymotion. Graticule 13 is formed around the entire circumference of member1 and comprises a transparent substrate having opaque markings formedthereon. By interrupting the light path between light source andphotocell these markings provide the desired pulses. Other means ofproviding suitable pulses may of course by used.

The rotatable member 1 carries a source 14 of penetrating radiation.This may be similar to the source described in U.S. Pat. No. 4,002,917in which a substantially point source 15 of X-rays 16 is scanned over anelongated anode 17 by the scanning of an incident electron beam by meansnot shown. The X-rays 16, which are confined to a fan-shaped spread bysuitable collimator means 18, are, after passage through the body,incident on a detector means 19. Detector means 19 will be described ingreater detail hereinafter.

Also carried on member 1 is the collimator arrangement 18. Thiscomprises in this example a plurality of thin parallel platecollimators, made of molybdenum or other suitable material, which arearranged to define the X-rays 16 into a narrow fan of X-rays directed atdetector 19 and having the same angular spread for all positions of thespot 15. Other collimators, not shown, restrict the X-rays to the planeof the slice to be examined. The collimators are shown in simplifiedform in FIG. 1 and in one practical example are plates two thousands ofan inch (mils) thick, 900 mills long and at 18 mils spacing. Consideringa typical dimension of X-ray spot 15 on the anode 17 of the tube 14 tobe 80 mils diameter it will be apparent that the fan of X-rays is formedby four or five collimator slits so that the motion of the X-ray fan, ondetector 19, in response to movement of spot 15 is substantially steady.It should be noted that the intensity distribution across the fan,produced by the collimator arrangement, should be taken into account inprocessing. A collimator arrangement of the dimensions given can be usedto give a fan of about 2° extent if placed at a suitable distance fromthe source. In this example of the invention a fan of substantially 1.8°is considered.

The detector means 19 comprises a strip of individual detectors such asscintillator crystals or photodiodes, lying in the plane of the slice tobe examined so as to intercept substantially all of the X-rays 16 forall positions of the spot 15. As shown in FIG. 1a this strip ofdetectors is only irradiated over a small portion at any time. For thepurposes of explanation it will be assumed that 3 cm of the strip is soirradiated at any time. The detector comprises detector elements eachcovering 1 mm of the strip so that thirty such elements supply dataacross the 3 cm of the fan. This data corresponds to thirty individualbeam paths in the fam. The entire detetor is typically 30 cm longincluding 300 detector elements. In this example the detectors arescintillator crystals co-operating with photomultipliers indicatedgenerally at 20.

In operation the X-ray spot is scanned steadily across the anode 17 oftube 14 and correspondingly the fan of X-rays 16 scans in a plane acrossbody 3 and surrounding materials and along detector strip 19. In thisexample the irradiated region of the detectors moves in the samedirection as and approximately parallel to the source spot 15 as aresult of the form of collimators 18 used. The outputs of the detectorelements are integrated for a period in which the irradiated region ofthe detectors is moved 1 mm so that each detector provides one datum fora respective beam path. For the immediately following integrationinterval the data are obtained for elements displaced one place in thedirection of scan i.e. with an extra element at one end of theirradiated region and one less at the other. The detector elementsirradiated are thus progressively changed as the scan progresses.

It can be seen by this means the information relating to any one smallregion of the body is obtained by many detectors so that the effect ofrelative detector errors is reduced.

To irradiate the body over a sufficient number of beam paths source 14and detector means 19 are in this example orbited about an axis 21 -perpendicular to the slice of the body 3 to be examined. This may beachieved in steps between each scan of the spot 15. However, since theangle of the fan is 1.8° this will be the required orbital movement forone lateral scan and it is sufficiently small to be provided by acontinuous orbital movement without significant distortion ormisplacement of the beam paths.

As mentioned hereinbefore, at any time only a small proportion of thedetector elements of detector means 19 are irradiated, typically thirtyout of three hundred. That situation is utilised in the detectorarrangement shown in FIG. 2. The figure shows, for the sake of clarity,a simplified arrangement for which detector means 19 comprises twentyfive detector elements, in the form of scintillation crystals, of whichonly five are irradiated at any time. The intensity of light emitted bythe scintillators is measured by five photomultipliers 20₁ to 20₅ eachof which receives light from five detector elements through individuallight guides 22. The light guides are represented in the Figure bysingle lines. However it will be understood that each light guide inpractice receives light from one entire face of a crystal, the otherfaces being silvered to prevent loss of light. The light guides 22 arearranged so that the photomultipliers receive light from detectorelements in interlaced manner. In this example each receives light fromelements spaced five positions apart. It will be seen from FIG. 2 that,although each photomultiplier receives light from five detectorelements, only one of these will be irradiated at any time. Thus, forthe position of X-ray fan 10 shown in FIG. 2, each photomultiplierreceives light along the first light guide from the right, at themultiplier, and no light along the others. As the fan moves one elementto the left only the light to photomultiplier 20₅ changes so that lightis received along the second light guide. It will be apparent that, inthis manner, the twenty five detector elements can be covered by thefive multipliers if the data from those multipliers is appropriatelyallocated in the processing used. Other numbers of detector elements andphotomulipliers may be used in a similar manner.

Instead of using five different photomulipliers a five, or more, channelphotomultiplier can be used. This may be of the type described in U.S.Pat. No. 3,872,337. This may equally well be a three hundred channelphotomuliplier if desired. In that case the photomultiplier can beplaced close to the detector elements with short, or no light guides sothat each element would at all times supply light to one photomultiplierchannel. However, since, as described above, only a small number ofdetector elements are irradiated at any time a grouping similar to thatof FIG. 2 can be effected by joining the photomultiplier channel anodesin groups internally. By this means the number of output connectionsrequired would be reduced, simplifying construction problems. As in theprevious example other numbers of irradiated elements and groupings maybe employed as desired.

It should be understood that FIG. 2 is illustrative of a manner oforganising the detector output. However other positions, for example ofphotomultipliers 20, can be adapted for convenience of construction.

As mentioned hereinbefore large numbers of detector elements areemployed in a practical arrangement, typically 300 arranged in sixcycles of fifty elements. Fifty detector elements, say 1 mm apart, canbe irradiated by the narrow fan of X-rays, the individual fifty beampaths being narrow enough to give the desired spatial resolution withinthe body. This results, however, in a large quantity of output data and,since the angular resolution so obtained is unnecessarily good, some ofthis data may be combined to give a reduced angular resolution. Thearrangement in this example is that data for beam paths passing throughsubstantially the same parts of the body should be combined. In practicethis means that data for each beam in the fan is combined with dataobtained from beams incident on a number of, say three, adjacentdetectors and passing through the same predetermined point in the body.A time delay of τ of seconds is applied between those adjacentdetectors. The delay τ is equal to the time which elapses between thepassage of a beam incident on one detector through the predeterminedpoint and the passage of a beam incident on the next detector throughthe same point. The data for the first detector is delayed τ seconds andadded to that of the second and the two are delayed by a further τseconds and added to the data for the third detector. The arrangement isassumed to be that employing continuous orbital motion thus the threebeam paths for which data is combined are not strictly parallel but givea composite beam path which is narrower at the centre of the body andslightly thicker at the edges. For three beam paths this does not givesignificant error but allows a reduction of storage to one third of thatotherwise required.

In an alternative mode of operation of apparatus such as that of FIG. 1the X-ray spot scan and collimators 18 can be arranged so that the faneffectively rotates about the body, with the region of irradiateddetectors moving laterally in the opposite direction to the source spot.If the extent of anode 17 and detectors 19 is sufficient the orbitalmotion may be dispensed with. In that case the organisation of the datacan be similar to that described in U.S. Pat. No. 4,206,359. As afurther alternative the scanning X-ray source can be replaced by aconventional source such as a rotating anode tube and the scan of thefan of X-rays relative to the detectors provided solely by orbitaland/or lateral scanning motions of that source.

It should be noted that, in the arrangements described, afterglow indetector elements no longer being irradiated can still be intercepted bythe photomultipliers and introduce some noise into the data. For thisreason scintillator crystals having low afterglow should be employed.The problem can be alleviated by the use of other detectors such assemiconductor diodes, which may be germanium photodiodes. Gas filledcounters or other detectors may also be used. In those cases thegrouping, if desired, may be of any suitable multiplexing of the outputsignals. Alternatively shutter means or similar may be provided tointercept the emitted light between the crystals and the associatedphotomultipliers.

FIG. 3 shows a development, of the arrangement described, for which theorbital movement, of X-ray source 14 and detector means 19, is notrequired. Aperture 2 is surrounded by a ring of individual scanningX-ray tubes 14 of which the glass envelopes, indicated at 23 arearranged to adjoin. Inside the ring of tubes 14 there is provided a ringof collimators 18. X-ray tubes 14 having anodes 17 and collimators 18are essentially similar to those described in relation to FIG. 1, tubes14 being fixed in relation to the body in aperture 2. Outside of tubes14 there is provided a further ring comprising a plurality of detectormeans 19 each of which is as described hereinbefore.

At any time one of the tubes 14 is in operation, the X-rays being formedinto a fan by collimators 18 nearest to the tube and thereafter passingthrough the body in aperture 2 to be received at a detector means 19opposite. It will be understood that for this purpose the ring ofdetector means 19 must be set in a sufficiently different plane fromtubes 14 for the X-rays to reach the detectors unobstructed. This is asource of slight error in the desired data but such errors largelycancel for the data obtained from the 180° displaced detector.Collimators 18 may be arranged so that the beam passes through themafter passing through the aperture 2 as well as before.

In operation the X-ray tubes are operated in sequence so that the X-rayspot on the anode orbits in effect around the body in aperture 2. Thering of collimators 18 is arranged to rotate around aperture 2 but at arelatively slower rate than the rotation of the FIG. 1 arrangement. Theangular velocity desired is such that the collimators move through anangle slightly less than the beam spread angle, of the fan of X-rays 16,during one revolution of the X-ray spot. In the example shown in FIG. 3the collimator is in ten sections so that, without rotation, the angleof the centre beam of the fan would change by 36° when the spot movesfrom one section to the next. For the 1.8° fan of the example thecollimators rotate 1.8° in one spot revolution so that, when the spotreturns to the same collimator section it begins to fill in the missing36°. Thus twenty revolutions of the spot are required to fill in allmissing values. The exect number used is tailored to give a suitabledegree of overlap between fan beams for adjacent positions to reducenoise problems. It will be apparent that this collimator movementchanges the fan position by 0.18° as it crosses each section but such asmall error can be disregarded. It should be noted that any detectorsnot being irradiated may be switched out of the circuit by any suitablemeans to reduce noise problems further.

In an alternative mode of operation of the FIG. 3 arrangement, the fanof radiation may be of sufficient extent to encompass the entire regionof interest in the body. In that case, in conjunction with larger sourceand detector sectors, the operation would be such that the position ofthe group of detectors irradiated by the fan orbits in effect about thebody in the same direction as the source spot.

FIG. 4 shows in simplified block diagrammatic form an arrangement forprocessing the output signals derived by the arrangement of FIG. 2disregarding the steps required for combining adjacent detector outputswith appropriate delays. The five photomultipliers 20₁ to 20₅ are shownalthough it will be understood that there may be a greater number ofphotomultipliers or outputs from a single photomultiplier. The signalsare amplified in amplifiers 24₁ to 24₅ and integrated and converted todigital form in converters 25₁ to 25₅. The integration period is asallowed by the progress of the scan of the X-ray source spot 15 and iscontrolled by signals from a scan control unit 26, which also controlsthe source spot 15. Scan control unit 26 also receives signals fromphotodetector unit 12, related to the progress of the orbital scan, sothat the scan of source spot 15 can be properly related to the orbitalmovement. The data are provided to appropriate locations in a store 28in response to an address selector 27. The locations in store 28 arechosen so that successive outputs from each photomultipler are appliedto storage locations representing beam paths at successive angles in thefan. After the fifth such angle, in this example, the data are appliedto a new location representing a parallel beam path again at the firstangle and the cycle recommenced. In this way the data are allocated tostorage locations representing five sets of data each for parallel beampaths at one of the angles of beams in the fan, the allocation takinginto account the grouping of outputs shown in FIG. 2. When store 28contains data for the complete sets of beam paths, properly sorted, thisdata is applied to a processing unit 29 for processing, for example, asdescribed in U.S. Pat. No. 3,984,129 or in U.S. Pat. No. 3,778,614. Theprocessing derives absorption values for individual elements of a matrixof elements notionally delineated in the planar slice being examined.The values are then provided as signals applied to correspondingelements of a representation on a display unit 30. Unit 30 may be acathode ray tube, line printer or other suitable output device.Alternatively it may be applied to permanent storage, not shown, forfuture display.

The apparatus described hereinbefore is intended to acquire all therequired data in a very short time, possibly as short as one hundredthof a second for the arrangement of FIG. 3. A suitable analogue todigital converter should be employed to meet such rates of acquisition.Such a connecter may operate in the known manner of converting theoutput of a digital counter to analogue form and counting up or down tomatch that ouptut to the input voltage. However it may be divided into aplurality of sections each to convert to digital form signals betweenpresent threshold levels, to operate at a faster rate.

It will be appreciated that the invention is not limited to the formsdescribed hereinbefore and that other arrangements may be devised.

What I claim is:
 1. A process comprising providing a stationary ring ofx-ray detectors, placing a body within the ring, irradiating the bodywith x-rays coming from an x-ray origin which moves around the bodyalong a ciruclar path having a radius different from that of thedetector ring, and deriving, from the detectors, at least signalsrelated to x-rays coming from the origin and reaching the detectorsafter passing through the body.
 2. A process as in claim 1 in which thex-ray detectors receive x-rays which have passed through a sectionalslice of the body.
 3. A process as in claim 2 in which the x-rays whichirradiate the body are collimated into a fan spanning the entire slice.4. A process as in claim 3 including using the signals derived from thedetectors to produce and display an x-ray image of the slice.
 5. Asystem comprising a stationary ring of x-ray detectors, means forsupporting a body within the ring, an origin of x-rays irradiating thebody and means for moving it around the body, along a circular pathwhich has a radius different from that of the detector ring, and meansfor deriving, from the detectors, at least signals related to the x-rayscoming from the origin and reaching the detectors after passing throughthe body.
 6. A system as in claim 5 in which the x-rays irradiate asectional slice of the body and including a processor using said signalsto produce an x-ray image of the slice.
 7. A system as in claim 6 inwhich the x-rays irradiating the body are in the form of a fan spanninghe entire slice.
 8. A system as in claim 7 including means fordisplaying selected characteristics of said image.
 9. A system as inclaim 7 in which the radius of the circular path along which the originmoves is greater than that of the detector ring.
 10. A system forexamining a transverse slice of a patient's body comprising:a ring ofdetectors of penetrating radiation surrounding the slice; and a sourcewhich irradiates, with penetrating radiation coming from an origin whichorbits around the slice and moves relative to the detector ring, boththe slice and the detectors which at the time are opposite the slicefrom the origin; wherein the detector ring and the origin orbit areradially spaced from each other by a substantially constant distance.11. A system as in claim 10 including a data acquisition system derivingfrom the detectors signals related to the penetrating radiationirradiating the detectors and a processor system using said signals tobuild up and display a picture of the slice.
 12. A system for examiningan object comprising:an array of detectors of penetrating radiationsurrounding a transverse slice of the object at least halfway; and asource which irradiates, with penetrating radiation coming from anorigin which moves relative to the array of detectors along at least onecurved path extending at least halfway around the slice, both the sliceand detectors which at the time are across the slice from the origin;wherein successive positions of the origin along said at least onecurved path are not spaced from each other by detectors.
 13. A system asin claim 12 including a data acquisition system deriving from thedetectors signals related to the penetrating radiation irradiating thedetectors and a processor system using said signals to build up anddisplay a picture of the slice.
 14. A system as in claim 12 in which thearray of detectors and the at least one curved orbit substantiallyconform to respective circles which have different radii.
 15. A systemas in claim 14 in which each of the array of detectors and the at leastone curved orbit substantially fully encircles the slice.
 16. A systemas in claim 14 in which the array of detectors and the at least onecurved orbit substantially conform to respective planes which are spacedfrom each other along an axis transverse to the slice.
 17. A system asin claim 16 in which said respective circles are co-axial and saidplanes are spaced along the common axis of the circles.
 18. A system asin claim 14 in which the radius of the circle to which the detectorarray conforms is greater than the radius of the circle to which the atleast one curved orbit of the origin conforms.
 19. A system as in claim12 in which the array of detectors and the at least one curved orbitsubstantially conform to respective planes which are spaced from eachother along an axis transverse to the slice.
 20. A system as in claim 12including collimators which collimate the radiation before the radiationimpinges on the slice into a fan-shaped beam which is co-extensive withthe slice.
 21. A system as in claim 12 in which the array of detectorsand the at least one curved orbit of the origin conform to respectiveradially spaced arcs which at least partly overlap each other in angularextent around the slice.
 22. A system as in claim 20 in which thedetectors generate respective outputs related to radiation impingingthereon, and including a data acquisition system which digitizes andstores at least selected ones of said detector outputs.
 23. A system asin claim 22 including a processor which uses the detector outputsdigitized by the data acquisition system for computerized reconstructionof an image of the slice.
 24. A method comprising:moving a radiationorigin along at least one curved path around an object to irradiate atransverse slice of the object with penetrating radiation fanning outfrom positions of the origin along said at least one curved path; anddetecting at different times the radiation at detectors which are atthose times across the slice from the origin and are a part of adetector array which extends at least halfway around the slice; whereinsaid successive positions of the origin are not spaced from each otherby detectors.
 25. A method as in claim 24 including deriving from thedetectors signals related to the penetrating radiation irradiating thedetectors and using said signals to build up and display a picture ofthe slice.
 26. A method as in claim 25 in which said object is apatient, and said picture is a picture of the x-ray response of theirradiated slice.
 27. A method as in claim 24 in which said patient is ahuman patient.
 28. A method as in claim 27 in which said pictureselectively shows both soft tissue and bone.
 29. A method as in claim 24in which the at least one curved path of the origin and the detectorssubstantially conform to respective coaxial arcs of different radiiwhich are axially spaced from each other.
 30. A method as in claim 29 inwhich each arc is more than 180°.
 31. A method comprising:irradiating atranverse slice of a body with a fan beam of penetrating radiationcoming from an origin which moves along an orbit surrounding the sliceat least halfway; detecting said fan beam after attenuation thereof bythe slice of a detector array which surrounds the slice at least halfwayto produce detector signals related to the radiation received by therespective detectors along respective directions; wherein successivepositions of the origin along its orbit are not spaced from each otherby detectors.
 32. A method as in claim 31 including correcting thesignals derived from the detectors on the basis of contributions fromothers of said signals, and cumulatively allocating the correctedsignals to picture elements which are along the directions correspondingto the respective signals to build up a picture of the slice.
 33. Amethod as in claim 28 including visually displaying said picture.
 34. Amethod as in claim 29 including storing said picture in or on a recordmedium.
 35. A method as in claim 28 in which both the detector array andthe orbit of the origin encircle the slice.
 36. A method as in claim 35in which the detector array has a greater radius than the orbit of theorigin.
 37. A method as in claim 36 in which the detector array and theorbit of the origin are offset from each other in a direction transverseto the plane of the orbit.
 38. A system comprising:a circle of detectorsof penetrating radiation completely surrounding a transverse slice of anobject; a source which irradiates, with a fan of penetrating radiationcoming from an origin which moves around the slice along a planar orbit,both the slice and the detectors which at the time are across the slicefrom the origin, to successively irradiate successive groups of saiddetectors, wherein successively irradiated groups differ from each otherby at least one detector but substantially overlap with each other. 39.A system as in claim 38 including means for deriving from the detectorssignals related to the penetrating radiation irradiating the detectorsand for using said signals to build up and display a picture of theslice.
 40. A system as in claim 39 including means for interconnectingthe detectors in sets of which not all detectors are irradiated from theorigin at any one time, and for providing the common output of a set asa single output channel representing a signal from an irradiateddetector.
 41. A system as in claim 38 including means for deriving fromsaid detectors signals each related to the attenuation of thepenetrating radiation along a respective direction through the slice andfor modifying each respective signal on the basis of signals for otherdirections distributed throughout substantially the entire slice and forcumulatively allocating for each respective picture element the modifiedsignals for those directions which pass through or adjacent the centerof the slice element corresponding thereto, to build up on picture ofthe slice.
 42. A system as in claim 41 including means for at least oneof storing and visually displaying said picture.
 43. A system as inclaim 38 in which the circle of detectors has a greater radius than theorbit along which the radiation origin moves.
 44. A system as in claim38 in which said fan is wide enough to encompass at least the entireslice.
 45. A system as in claim 38 in which the detectors do not movearound the slice.
 46. A method of examining an object with penetratingradiation comprising the steps of:irradiating at least one slice of anobject with penetrating radiation coming from positions which are alongat least one curved path; and detecting object-attenuated radiation withat least one curved detector which has substantially uniformly spacedindividual detector elements encircling the slice at least halfway. 47.A method as in claim 46 including collimating the radiation coming fromsaid positions along the curved path before it impinges on the sliceinto at least one fan-shaped beam
 48. A method as in claim 47 in whichsaid at least one fanshaped beam is wide enough to encompass the slice.49. A method as in claim 48 in which the detector elements generateoutputs related to radiation impinging thereon, and including digitizingsaid detector outputs.
 50. A method as in claim 49 including storing thedigitized detector outputs prior to further processing thereof.
 51. Amethod as in claim 49 including using the digitized detector outputs forcomputerized reconstruction of an image of the slice.
 52. A method as inclaim 51 including displaying said image of the slice.
 53. A method asin claim 51 in which the image can contain a wide range of image values,and including displaying at any one time only a selected window of saidimage values which is at a selected level of said values, wherein therange of values in said window is narrower than said wide range.
 54. Asystem for examining an object with penetrating radiation comprising:asource irradiating at least one slice of an object with penetratingradiation coming from positions which are along at least one curvedpath; and at least one curved detector which comprises substantiallyuniformly spaced individual detector elements and encirlces the slice atleast halfway to detect object-attenuated radiation.
 55. A system as inclaim 54 including a pre-object collimator which collimates theradiation coming from said positions along the curved path into at leastone fan-shaped beam.
 56. A system as in claim 55 in which said at leastone fanshaped beam is wide enough to encompass the slice.
 57. A systemas in claim 56 in which the detector elements generate outputs relatedto radiation impinging thereon, and including a data acquisition systemwhich digitizes said detector outputs.
 58. A system as in claim 57including a store for storing the detector outputs digitized by the dataacquisition system.
 59. A system as in claim 57 including a processorwhich uses the detector outputs digitized by said data acquisitionsystem for computerized reconstruction of an image of the slice.
 60. Asystem as in claim 59 including displaying said image of the slice. 61.A system as in claim 59 in which the image can contain a wide range ofimage values, and including displaying at any one time only a selectedwindow of said image values which is at a selected level of said values.